Hydraulic valves for controlling the movement and position of hydraulic actuators that are connected to large loads usually include several hydraulic circuit protection devices necessary to prevent damage to the hydraulic system, either the actuator or the hydraulic valves themselves. The two primary problems faced by hydraulic systems are that a sudden impact on the actuator or a sudden application of high pressure hydraulic fluid may lead to a large high pressure pulse in hydraulic components that are not sized to handle these high pressure pulses. To cure this problem, hydraulic controls and particularly spool valves that are commonly used to regulate hydraulic flow are equipped with an over-pressure relief circuit that dumps excess pressure back to the hydraulic tank, which is at a substantially lower pressure than the hydraulic supply pressure. Typical tank pressures range from 0 to 100 psi, where typical supply pressures may range from 500 to 4,000 psi. The relief valve, by opening, permits fluid pressure applied to the actuator to be automatically reduced. Once the pressure is within the proper range, typically 100 to 800 psi, these over-pressure relief valves automatically close.
Another problem faced by hydraulic systems is the formation of a vacuum in hydraulic lines. Just as hydraulic over-pressure can damage hydraulic systems by bursting actuators, valves and conduits, a vacuum in a hydraulic line can cause the hydraulic fluid to vaporize. These vapor bubbles in themselves are not damaging. When the pressure is increased, however, these bubbles collapse upon themselves as the hydraulic vapor condenses. There are substantial local transient pressure waves produced. Pressure waves formed by the collapsing bubbles will, over time, damage and dangerously weaken the hydraulic components in the system. This problem is called “cavitation”.
For this reason, hydraulic controls, and particularly hydraulic spool valves and valve bodies, are provided with “anti-cavitation valves”. These valves operate in a somewhat similar fashion to over-pressure relief valves. In a sense, anti-cavitation valves are under-pressure relief valves. When a hydraulic pressure drops below tank (or “return”) pressure, the anti-cavitation valves automatically open and permit the flow of hydraulic fluid into the low pressure regions, thus preventing the formation of hydraulic vapor bubbles. When the under-pressure condition is relieved, the anti-cavitation valves automatically close, thereby cutting off additional hydraulic flow.
Another common feature in hydraulic controls, spool valves and valve bodies is the hydraulic check valve. A check valve is one that permits the flow of fluid in one direction only. These valves are typically disposed between a manually or electrically actuated spool (that direct flow to an actuator) and the actuator itself. Check valves relieve the pressure differential that would otherwise remain on the spool at all times. Without the check valve, sudden over-pressure conditions in the actuator would be instantly transmitted backwards to the control valve that regulates flow to or from the cylinder. These sudden pressure pulses can cause the control valve (the directional spool valve) damage. In addition, the check valves reduce leakage that would otherwise occur if the actuator pressure was maintained on the spool.
In prior art spool valves, these three valves: check valve, anti-cavitation valve and over-pressure relief valve, typically required that three different openings be drilled into the valve body, one for each valve. This required extensive machining. Typically, the valve body was drilled at three different locations.
What is needed, therefore, is a new check valve, over-pressure relief valve, and anti-cavitation valve arrangement that reduces the required or typical number of holes in a valve body. It is an object of this invention to provide such a valve arrangement.